1. Field of the Invention
The invention concerns techniques for detecting and accurately determining the location of a physical disturbance. Input signals are coupled in two opposite directions along a waveguide, through multiple signal paths, at least one such path traversing a detection zone. The waveguide, for example, can consist of one or more optical fibers. Multiple signal paths can be provided by distinct signals traveling in different fibers, and/or multiple signal paths can be provided using different modes of signal propagation in one or more of the same fibers.
In exemplary applications of the inventive techniques, optical fibers are routed around a security perimeter, along or across a road or path, coextensively with a power or signal transmission line, on or near a pipeline, etc. A disturbance such as sound or vibration from nearby activity changes the propagation conditions of the light signals carried in the multiple signal paths, simultaneously locally affecting the signal paths in both opposite directions. A phase relationship change occurs for the signals carried along the multiple signal paths in each of the two opposite directions. The change is carried along in the signals propagating away from the point of the disturbance in both directions. The propagation time difference, between appearances of the corresponding changes in the phase relationship at each end, is determined and used to resolve the location.
According to an inventive aspect, the disturbance is detected and located from a time variation in phase relationship between the signals carried along different signal paths and for each of the counter-propagating signal directions. A phase responsive receiver is used to obtain the phase relationship between signals on different signal paths. The phase responsive receiver comprises at least one beam combiner and at least two detectors to mix and to detect the signals from at least two signal paths, respectively.
Preferably, the beam combiner, such as a bidirectional coupler, functions as a beam splitter for producing multiple input signals paths in one direction and also forms an optical interference node at the receiving end for the signals propagating in the opposite direction. The arrangement can be symmetrical, with couplers at each of the ends splitting signals into multiple paths directed toward the opposite ends, while receiving and interfering the signals from the opposite end. Through the beam combiner, the received-and-interfered signals produce at least two phase-related intensity responses for each of the phase responsive receivers. The two intensity responses provide independent phase-related variable values when applied to detectors. These values can be used to obtain the phase relationship between signals carried along different signal paths. Two phase relationship signals are obtained, preferably as successions of data samples representing phase versus time, for the signals in each of the opposite directions, as affected by the disturbance.
The beam combiner can be a three-by-three fused fiber coupler, or an n-by-m coupler, a two-by-two coupler with polarization dependent elements, or multiple cascaded couplers. The light levels at the detectors are sampled and processed by techniques involving at least two independent phase-related variables, modeled and preferably normalized and reoriented using multi-dimensional data analysis techniques as described herein. The techniques discriminate for disturbance-induced variations in phase relationship, as a function of time, for each of the counter-propagating directions. A correlation function then matches the corresponding variations of phase versus time for the opposite directions, deriving a differential propagation delay. The differential delay enables accurate resolution of the location of the physical disturbance.
2. Prior Art
A security system should detect and provide information about any intrusion into a protected area or facility. An advantageous system should discreetly detect even modest physical disturbances, and report the location of the disturbance so as to permit corrective action to ensue promptly.
One technique for locating a disturbance is by determining the difference in timing between the arrivals of effects of the disturbance, in two counter-propagating signals that are both affected by the disturbance. A relative delay in arrival of the disturbance induced effects in the signal propagating in one direction versus the other direction indicates a longer propagation distance from the disturbance to the receiver, where the signal is detected. Measuring the delay can permit one to calculate an apparent location of the disturbance. This technique is described for example, in British Patent GB 1,497,995—Ramsay, entitled “Fiber Optic Acoustic Monitoring Arrangement.”
Optical fiber has inherent advantages as a waveguide, such as low loss, immunity to electromagnetic noise and other characteristics, which are useful in remote sensing. The measurement of the disturbance effects in Ramsay utilizes an interferometer or interference sensor. An example of an interference sensor is the Mach-Zehnder interferometer, which has been applied to acoustic sensing, magnetic sensing, temperature sensing, pressure sensing, structure monitoring, etc, including using optical fibers, as described in “Overview of Mach-Zehnder Sensor Technology and Applications” by Anthony Dandridge and Alan D. Kersey, Fiber Optic and Laser Sensors VI, Proc. SPIE Vol. 985, pp. 34-52 (1988).
In addition to GB 1,497,995—Ramsay, cited above, the publication “Fiber Optic Distributed Sensor in Mach-Zehnder Interferometer Configuration” by Bogdan Kizlik, TCSET'2002 Lviv-Slavsko, Ukraine, proposes a similar location resolving technique. Recent U.S. Pat. Nos. 6,621,947 and 6,778,717 describe a perimeter defense system also based on this principle.
These prior art teachings produce an interference intensity signal versus time for each of two opposite signal paths, and seek to determine the location of the disturbance from the difference in propagation time over two counter-propagating signal paths, between the appearances of corresponding time variations at receiving ends for the opposite signal paths. There are problems, however, when attempting to use optical fiber waveguides and the like for location detection in this way. Polarization induced effects can reduce or defeat the usefulness of these prior techniques for discerning the location of the disturbance.
Light waves interfere only when there is some correspondence in the state of polarization, permitting the beams to interfere. Two light waves that are orthogonally polarized cannot interfere. Over plural paths between a light source and two or more detectors, the birefringence of the fibers forming an optical path can change the state of polarization and phase characteristics of the light beams. The birefringence of an optical fiber may be small compared to its refractive index. Nevertheless, an accumulated polarization effect arises, particularly over a long distance. Prior art systems cannot perform consistently, and in some circumstances do not perform at all, because the interfering optical beams vary from time to time between more or less parallel and more or less orthogonal states.
Variable beam interference conditions caused by polarization state changes are recognized as a problem in single light path interferometers, the problem being known as polarization-induced fading. The problem is described, for example, in “Polarization-Induced Fading in Fiber-Optic Sensor Arrays” (Moshe Tur, Yuval S. Boger, and H. J. Shaw, Journal of Lightwave Technology, Vol. 13, No. 7, p 1269, 1995). This publication seeks to enhance the visibility of the interference beam in a single-channel fiber based interferometer, where the light travels along a single direction.
Polarization induced phase shift, which is caused by the mismatch of the polarization of the interfering beams, is a somewhat different effect from polarization induced fading, but causes measurement problems, because polarization induced phase shift can be difficult to distinguish from other factors. If there is a polarization induced phase shift, the interference intensity signals at the detectors for the two counter propagating signals may not correlate closely. The technique of calculating a location for the disturbance relies on identifying two corresponding variations in amplitude over time, and then measuring the difference in time of arrival between the two counter-propagating signals. Such a measurement is difficult and potentially inaccurate, if variations in the two signals cannot be properly matched.
In the prior art interferometer system, signal phase conditions are varied by the disturbance to produce variations in interference amplitude. But the swing in the interference output signal is not exclusively or linearly related to the change in relative phase caused by the disturbance. The interference amplitude is affected by changes in polarization states which generally are different for the two signal directions because of differences in the polarization effects in the two counter propagating directions. The interference amplitude is not uniquely related to the relative phase relationship between interfering beam along different paths. For these reasons, a disturbance locating security system as in GB 1,497,995—Ramsay may be undependable or may need regular polarization adjustment. Measurement failure from polarization induced effects is an imminent danger. The correlation of time varying interference signal signatures for a given disturbance for the two opposite signal paths produces uncertain location measurements due to unpredictable polarization effects. For all these reasons, the system dependability and accuracy are less than might be desired for security purposes.
In terms of structure, the prior art technique for coupling signals typically employs two-by-two optical couplers, such as fused fiber junctions, for splitting and/or for combining light signals.
It would be advantageous for the location detection purposes discussed, to enable an accurate determination of phase variations between two received signals applied as two inputs to a coupler, and to do so free of complications from polarization fading and phase shift. What is needed is additional independent variable information whereby the two independent output variables can be derived to permit the effects of phase to be discriminated from the effects of polarization.
The present invention avoids detrimental effects of polarization induced fading and phase shift. Conditions are established that provide a robust response notwithstanding time changing polarization transformation characteristics such as birefringence. In certain embodiments, these conditions are established by providing coupler outputs that are characterized by a phase difference, permitting an analysis with the benefit of at least two and optionally additional independent variables by which phase effects are discriminated from polarization effects. A multi-dimensional data analysis technique is used, as illustrated by optional techniques in the disclosure, demonstrating how independent variables are translated substantially exclusively to phase angle as a function of time. The adverse effects caused by polarization are reduced to signal to noise ratio effects and can be readily avoided. The invention is practical, dependable and effective in perimeter security systems, as well as in other distributed sensing purposes.